So, using 1 stamp of value 1, the stamp value is 1.
Using 2 stamps of value 2, the stamp value is 2.
Using 3 stamps of value 1, the stamp value is 3.
Using stamps of denomination a, the lowest stamp value will be a.
Now, using 2 stamps 1 of value a and the other of 1, a stamp value of a + 1 is obtained.
Then, using 3 stamps, 1 of value a and 2 of value 1 each, a stamp value of a + 2 is obtained.
Then, using 2 stamps of value a each, a stamp value of 2a is achieved.
Then, using 2 stamps of value a each and 1 of value 1, a stamp value of 2a + 1 is achieved.
As lowest integer has been asked for, we need not worry about 3a.
Now, putting a = 2, stamp values of 2, 3, 4, 4 and 5 are obtained.
Putting a = 3, stamp values of 3, 4, 5, 6 and 7 are achieved.
Putting a = 4, stamp values of 4, 5, 6, and 9 are achieved.
However, if a = 4, then using stamps of denomination 1, stamp values of 1, 2 and 3 can be achieved as explained before.
However, if a = 4, the lowest stamp value of 7 cannot be achieved.
Thus, the least possible integer for which there is no stamp value is 7.